Objective: Why is low resting heart rate (HR) associated with antisocial behavior (ASB), i.e., aggression and rule breaking, in adolescence? Theory suggests that personality traits mediate this relationship but differently with age. In the present study this age-effect hypothesis is tested; we expected that the relationship between HR and aggression would be mediated in preadolescence by the personality trait behavioral inhibition, but not by sensation seeking. However, the relationship between HR and rule breaking in adolescence was predicted to be mediated by sensation seeking, but not by behavioral inhibition. Hypotheses were tested separately for boys and girls. Method: HR in supine position was assessed in respondents to the TRacking Adolescents' Individual Lives Survey (TRAILS) (N = 1,752; 48.5% boys) at age 11 years. Rule breaking and aggression at age 16 were assessed with two subscales from the Youth Self Report (YSR) questionnaire. Personality (i.e., sensation seeking and behavioral inhibition) was measured at ages 11, 13.5, and 16 with the Early Adolescent Temperament Questionnaire-Revised (EATQ-R), Behavioral Inhibition System/Behavioral Activation System (BIS/BAS) scales, or NEO Personality-Index Revised (NEO-PI-R). Results: In boys, lower HR was associated with aggression and rule breaking in adolescence. The association between HR and rule breaking was mediated by sensation seeking in adolescence but not in preadolescence. Girls' HR was not associated with ASB, and no mediating effects were found. Conclusions: Our findings support the age-effect hypothesis in boys' rule breaking behavior. This shows that the association between HR and ASB depends on age, gender, and subtype of ASB. (Contains 3 figures and 1 table.)